This invention relates primarily to a process and apparatus for screening the intake stream of municipal wastewater treatment plants where the apparatus would be used at the influent to such plants to capture, wash and dewater fibers, plastics, textiles and other solids or objects which could plug, foul or contaminate downstream pumps, treatment equipment or processes.
The presence of undesirable, untreatable solids in municipal wastewater has been a problem ever since centralized treatment plants have been in existence, due to their tendency to clog pumps and foul treatment equipment and processes. These solids typically include plastics, textiles, fibers and other non-degradable, inorganic materials in excess of 0.25xe2x80x3 in size. In order to minimize the operational problems created by these materials, there has always been a strong desire for their removal at the inlet to the treatment plants, before they have an opportunity to create operational problems with subsequent equipment and processes.
Early screening devices primarily consist of a series of parallel bars spaced approximately 1xe2x80x2 or more apart, commonly referred to as a bar rack, which extended vertically downward to the channel floor. As the wastewater passed between the individual bars of the bar rack, solids which were larger than the spacing between the screen bars were captured on the upstream surface of the bar rack for subsequent removal with a hand rake. At that time, the solids, or screenings, were typically carried away in buckets then buried nearby.
As technology advanced, mechanical devices were developed to automatically rake the captured solids off the bar racks. These devices were most often either a single rake suspended at either end by cables and powered by a reversing winch, or multiple rakes attached at either end to two endless loops of chain which were powered by an electric motor. In the last 15 years, the cables and chains have been replaced by a single rake suspended on long arms from a motor-driven carriage traveling up and down the screen side frames above the influent channel. In all cases, the solids were dragged up the bar rack by the moving rake and deposited into a hopper or onto the concrete floor behind the screen for eventual pick-up and disposal.
Increasingly stringent effluent quality standards for wastewater treatment plants have necessitated the incorporation of more sophisticated treatment processes, which are more prone to malfunction due to plugging or fouling by foreign materials.
Thus, the most recent trend in screening equipment has been toward fine screens to remove any solids larger than xc2xc inch from the wastewater stream. These screening mechanisms typically consist of a series of screen panels hinged together end-to-end to form a continuous loop, somewhat like a conveyor belt. The individual panels are comprised of a series of thin parallel bars spaced xc2xc inch or more apart and a hinge rod to connect the individual panels together. The entire assembly is positioned vertically in a channel such that the solids entrained in the waste stream are captured on the upstream side of the screen panels as the wastewater passes through the mechanism. The travel of the screen panels causes the captured solids to be conveyed up and out of the channel where they are discharged.
At the same time the benefits of fine screening were being recognized, operators of the landfills where the solids, or screenings, were being deposited began demanding a cleaner, dryer product in order to avoid any potential liability associated with the screenings, should they be considered hazardous materials by the regulating authorities. The two trends were at direct odds in that the fine screens captured significantly more solids with a high water content as well as a large amount of putrescible material, which created both odor problems and potential health risks. In order to accommodate both situations, new machines were developed to wash and dewater the captured screenings. These machines typically incorporate combinations of water sprays, spinning impellers, scrub brushes and reversing screw augers and are quite complex, difficult to maintain and do not produce a clean, dry product.
There are numerous disadvantages to the present fine screening/washing/dewatering systems including:
1. The screening devices incorporate a multitude of articulating joints and rollers which are subject to the abrasive and corrosive elements of the wastewater, resulting in high operation and maintenance costs.
2. The screening mechanism themselves create considerable blockages of channel flow, creating significant head losses across the screen and resulting in higher pumping costs at the plant influent.
3. The need to substantially oversize the screen in order to pass a given amount of flow without exceeding recommended velocities through the screening panels.
4. The difficulty of discharging captured materials from the screen panels, resulting in the solids being xe2x80x9cconveyedxe2x80x9d past the discharge area and back down into the influent channel to be swept away by the fluid passing through the screen.
5. The accumulation of solids in the mechanical workings of the screening mechanism resulting in mechanical failures and the subsequent need for corrective maintenance.
6. The accumulation of solids on wipers, brushes and cross-members in the discharge area.
7. The spillage of solids onto the floor and other equipment around the screen create safety and health hazards as well as attract nuisance pests such as flies, rodents, etc.
8. The difficulty and expense of accommodating influent channels which are several feet below grade, requiring very tall, costly screens to elevate the screenings to the point of discharge.
9. The susceptibility of damage to the screen drive in deep pits due to flooding.
10. The need to provide some method of screenings conveyance between the screen""s discharge and the dumpster or washing/dewatering mechanism, which could be quite difficult and costly, depending on their location.
11. The operating, maintenance and housekeeping problems associated with belt conveyors and screw conveyors used in screenings transfer applications.
12. The complexity and expense of screenings washers and presses which presently incorporate motorized impellers, high pressure water sprays, rotating brushes and reversing augers.
In the present invention, the liquid to be treated enters the screening mechanism through the upstream end of the device where it is forced to pass through a screen panel before exiting the device through its downstream end. Solids too large to pass through the openings in the screen panel are retained on its upstream surface, from which they are subsequently removed by a vacuum header which traverses the screen panel. The captured solids are drawn into the vacuum header by the suction forces created by a solids handling pump, then pass through the pump and are subsequently discharged through a closed piping system to a washing/dewatering mechanism.
The initial section of the washing/dewatering portion of the invention, later referred to as the washbox, utilizes a series of staggered baffles in a closed chamber to create turbulence in the liquid containing the captured solids as it flows from one end of the chamber to the other, resulting in the degradation of organic materials and the washing of other materials. The outlet of the washing compartment of the device is in direct communication with the inlet of the dewatering aspect of the invention. Here, the captured solids flow into a chamber lined with fine screen panels, whereby the oversized solids are retained on the surface of the panels and the carrying fluid with finer particles passes through for subsequent return to the original channel. The rotating action of a screw auger passing through the dewatering chamber in close proximity to the fine screen panels scrapes the oversized solids from the surface of the fine screen panels and conveys them into and through a closed-sided, open-ended tube forming a compression chamber. Here, the excess liquid is forced out of the solids by the compressive forces created when the rotating auger forces the solids just removed from the screen panels in the dewatering chamber up against previously captured solids passing out of the compression chamber.
It is an object of this invention to provide a simplified screening/washing/dewatering system whereby the total number of moving parts requiring maintenance are minimized and system performance is maximized. It is also an object of this invention to provide a clean, odor-free screening/washing/dewatering system in which the undesirable solids removed from the fluid stream being treated are contained within a closed system from their point of capture, in the treatment plant influent channel, to the point at which the washed and dewatered screenings are discharged to a container for final disposal.
Other objects and advantages are:
To provide a screen panel having an open area of at least 85% to minimize the required size of the screening mechanism, the headloss through the screen, and the resulting capital and operational costs of the screening system.
To provide a positive means of removing the captured screenings from the screen panel and minimize the possibility for screenings carry-over to downstream processes and equipment.
To utilize, preferably, screen panels which are very deep in the direction of fluid flow in order to minimize the tendency of fibrous material to wrap around the individual screen plates and blind the open area of the screen panel.
To utilize a solids handling pump or eductor to create the vacuuming action necessary to draw the captured solids off of the screen panel and into vacuum header and to convey them through a closed pipe to the location of the screenings washing/dewatering device for subsequent processing.
To utilize a closed conduit to convey the screenings from the screening mechanism to wherever the washing/dewatering mechanism is located which could be any distance away, either horizontally or vertically.
To provide the ability to provide a screen panel with openings of less than xc2xcxe2x80x3 for the removal of smaller solids from waste streams without necessitating a change of any other components of the screening/washing/dewatering system.
Other features, advantages, and objects of the present invention will become apparent with reference to the following description and accompanying drawings.